Series Resistor Calculator
Resistors wired in series share a single current path, so their values simply add up. Enter each resistance below, pick a unit, and optionally add a supply voltage to also get the loop current and the voltage drop and power dissipated by every resistor.
Formula
Worked example
Three resistors of 100 Ω, 220 Ω and 330 Ω in series: R = 100 + 220 + 330 = 650 Ω. With 9 V applied, the loop current is I = V ÷ R = 9 ÷ 650 = 0.0138 A (13.8 mA). The 330 Ω resistor drops V = I·R = 0.0138 × 330 = 4.57 V and dissipates P = I²R = 0.0138² × 330 = 0.063 W. Total power is 9 × 0.0138 = 0.125 W.
How series resistance works
When resistors are connected end to end in a single line, they are said to be in series. Because there is only one path for charge to follow, the same current flows through each resistor in turn. The total opposition to that current is found by adding every individual resistance together, which is why a series chain always has more resistance than any one of its members. This additive rule comes straight from Ohm's law applied around the loop: the supply voltage equals the sum of the voltage drops, and dividing through by the shared current leaves the resistances adding directly. This calculator reads your list in ohms, kilo-ohms or mega-ohms and shows the total auto-scaled to a readable unit.
Voltage division, current and power
Series resistors are the basis of the voltage divider, one of the most common building blocks in electronics. Add a supply voltage and the calculator finds the loop current with Ohm's law (I = V ÷ R), then works out the voltage each resistor drops (V_i = I·R_i) and the power each dissipates (P_i = I²·R_i). Because every resistor carries the same current, each one drops a portion of the source voltage in direct proportion to its share of the total resistance, so two equal resistors split a supply in half while a 1:3 ratio splits it into a quarter and three-quarters. The per-resistor table makes this split explicit, and the largest resistor in the chain always dissipates the most heat.
Tolerance, power rating and real components
Real resistors carry a tolerance band, commonly 1% or 5%, so a measured total may differ slightly from the calculated value. Choosing a tolerance shows the worst-case spread of the total if every resistor sat at the same end of its band, though in practice the errors of many resistors in series tend to partially average out. Each resistor dissipates power equal to I²R, so always confirm that every resistor's power rating (for example a common 1/4 W part) exceeds the dissipation shown in the breakdown table. This calculator assumes ideal, purely resistive components at steady state; at high frequencies stray inductance and capacitance can shift the effective impedance.
Series vs. parallel at a glance
| Connection | Rule | Two 100 Ω resistors | Result vs. members |
|---|---|---|---|
| Series | R = R₁ + R₂ | 200 Ω | Larger than either |
| Parallel | 1/R = 1/R₁ + 1/R₂ | 50 Ω | Smaller than either |
Two 100 Ω resistors behave very differently depending on how they are wired.
Frequently asked questions
How do you calculate total resistance in series?
Add every resistance together: R = R₁ + R₂ + … + Rₙ. Because series resistors share one current path, their values sum directly, and the total is always greater than the largest individual resistor.
How do I find the voltage across each resistor?
First find the loop current with Ohm's law, I = V ÷ R_total, then multiply by each resistance: V_i = I × R_i. The drops are proportional to each resistor's share of the total, which is exactly how a voltage divider works. Turn on a supply voltage here to see every drop in the breakdown table.
How much power does each resistor dissipate?
Each resistor dissipates P = I²R, where I is the shared loop current. Because the current is the same everywhere, the largest resistor dissipates the most heat. Make sure every resistor's wattage rating (often 1/4 W) is above the value shown so it does not overheat.
Do the resistor values have to be in ohms?
No. Pick ohms, kilo-ohms or mega-ohms from the unit selector and enter your list in that unit, for example 4.7 in kΩ mode is 4,700 Ω. The total and breakdown are auto-scaled to a readable unit.